Method for producing investment castings

ABSTRACT

A method for producing investment castings, preferably in ceramic moulds, from alloys based on Al, Mg, Cu, Zn and Fe, characterised in that the ceramic mould is baked at temperatures of 800-1000° C. for 2-4 hours, then the mould is cooled to a temperature of 20-950° C. and is held at this temperature for 10-40 minutes, then it is poured with liquid alloy overheated by from 50 to 200° C. above the initial melting point and after a lapse of 10-100 seconds, the mould is immersed at a fixed or variable speed in a liquid cooling medium, which is a 1-99 volume percent aqueous solution of liquid polymer at a temperature of 15-85° C. 
     The liquid polymer is a polymer of the PAG or PVP, or ACR, or PEO type. The ceramic moulds made of aluminosilicate, or high alumina refractory materials, especially based on synthetic sand, for example Molochite, are used.

This invention relates to a method for producing investment castings,preferably in ceramic moulds, from alloys based on aluminium, magnesium,copper, zinc and iron.

After pouring the ceramic mould cavity with liquid alloy, a rapid, bulkor directional, cooling of this alloy is required to get a compactcasting without porosity, characterised by relatively homogeneous andfine-grain structure and, consequently, by high mechanical properties.

The ceramic moulds used in investment casting are characterised by lowthermal conductivity and therefore the time of the liquid metalsolidification is relatively long. As a result of the slowsolidification, a coarse-grain structure is formed, which is the causeof reduced mechanical properties of the casting.

To obtain the required compact structure in castings, a directionalsolidification of the metal should be provided. For example, a method toproduce investment castings according to patent specification DE-OS3629079 consists in this that it uses ceramic moulds with “pockets” madein the selected parts of mould. Before pouring of mould with liquidmetal, these pockets are filled with steel shot. Because of the heatcapacity of the steel shot considerably higher than the heat capacity ofthe aluminium alloy, an intense heat transfer from the liquid metal isthereby provided, which allows making castings with the areas ofdirectional solidification. In patent specification EP571703, as acooling medium for the ceramic mould poured with liquid alloy a fluidcoolant is used, preferably in liquid state, at varying degrees ofoverheating, wherein the boiling point is lower than the temperature ofthe metal poured. The liquid coolant should have an appropriateviscosity, and it is a mixture of substances characterised by differentboiling points. As an example, a composition comprising wax, glycol,ester and/or oil is given. Due to the flammability of the components ofthe coolant, the process is carried out in a closed container underinert gas atmosphere. According to patent specification U.S. Pat. No.6,622,774, ceramic moulds poured with aluminium alloy are cooled in anoil bath, which has a high flash point and low viscosity. On the otherhand, in patent specification US 2008/0011442, the ceramic foundry mouldfilled with liquid metal is kept at a pressure of at least 20 MPa forthe time of about 300 seconds, and then it is introduced into a liquidcoolant at a temperature of −100° C., or lower. It is advantageous tosubject the solidifying metal to the effect of ultrasounds, or toanother alternative treatment.

A method for producing investment castings according to the invention,preferably in ceramic moulds, from alloys based on Al, Mg, Cu, Zn and Fecharacterised in that the ceramic mould is baked at a temperature of800-1000° C. for 2-4 hours, is cooled next to a temperature in the rangeof 20-950° C., and is held at this temperature for 10-40 minutes, andafter a lapse of this time, the said mould is poured with liquid alloyoverheated by 50 to 200° C. above the initial melting point, and after alapse of 10-100 seconds, it is immersed at a fixed or variable speed ina liquid cooling medium, the said liquid cooling medium being a 1-99volume percent aqueous solution of liquid polymer at a temperature inthe range of 15-85° C.

Preferably, the liquid polymer is a polymer of the PAG, or PVP, or ACR,or PEO type. Preferably, the ceramic moulds made from thealuminosilicate or high alumina refractories, especially based onsynthetic sand, for example Molochite, are used.

Liquid polymers are solutions, suspensions or alloys of polymers, whichbelong to a particular group of liquids characterised by a measurableshape stability, reveal some characteristics of a solid body, and arecharacterised by a measurable elasticity. They represent polymolecularcarbon and hydrogen compounds, containing also oxygen, nitrogen,phosphorus and sulphur, as well as modifiers, inhibitors and otheradditives. Liquid polymers exhibit non-Newtonian fluid properties athigh shear rates, which means that their flow curves are not straightlines.

The applied cooling medium has nearly 2-times higher specific heat thanthe quenching oils, owing to which the temperature increase of themedium will be for a given batch weight reduced by approximately onehalf.

The aqueous solution of liquid polymers penetrating through the ceramicmould walls forms in contact with molten metal a thin separating polymercoating, which enables making castings with excellent surface quality.Applying the method according to the invention, the outer surfaces ofcastings are not contaminated with oil or wax, and therefore they do notrequire degreasing or other cleaning processes. The residues of thecooling medium that can remain on castings when the said medium occursin high concentrations in the cooling bath are not carbonised, but fullydecompose at high temperatures to form water vapour and carbon oxides.The cooling rate depends on the type of the polymer, on itsconcentration, and on the temperature of the aqueous solution. Theapplied cooling medium is non-flammable and environmentally friendly.Due to these characteristics, for the implementation of the methodaccording to the present invention, no sealed space is required but asucking-off installation is sufficient, since larger amounts of smokecan arise only in the case of the premature removal of moulds from thecooling medium.

Casting immersed in the polymer coolant crystallises and solidifies in abottom-up manner, and the down-gate crystallises as the last part of thecasting, performing at the same time the role of a feeding system. Rapidbulk crystallisation and cooling from the liquid state occur when thewhole ceramic mould, poured with liquid alloy, is immediately immersedin the cooling medium, this technique being applied to castings ofpractically equal wall thickness and smooth transition from one sectionto another. Rapid directional crystallisation occurs when the ceramicmould poured with liquid alloy is immersed at a constant or variablespeed in the liquid cooling medium.

Ceramic moulds made of aluminosilicate or high-alumina refractories arecharacterised by stable properties during changes in temperature. Highertemperature of the ceramic mould favours alloy castability, and thusenables making thin wall or ultra-thin wall castings.

Castings made by the method according to the invention have very goodquality of the outer surface, characterised by a low degree ofroughness, by glossy appearance, and by absence of defects of the gasmicroporosity type. The directional solidification provides goodinternal compactness, measured by density. Moreover, the castings arecharacterised by high homogeneity of the macro- and microstructure inthe wall cross-sections, the thicknesses of which are not greater than2-3 times. Their structure is more refined with respect to similarcastings solidifying according to the traditional methods of theinvestment casting process, in particular, the average values of thesecondary dendrite arm spacing are reduced, and in eutectic alloys afine-grain eutectic is formed. All these factors affect the increase ofstrength parameters R_(m) and R_(ρ0.2) and ductility A₅ in the tensiletest, while the magnitude of these parameters depends on the alloy typeand cooling conditions.

Examples of practical embodiments of the method for producing investmentcastings according to the invention are given below.

Example 1

A ceramic mould for a pilot conical casting of ø20×ø30×100 mm is made ofquartz flour and silica sand with SiO₂ content above 90 weight percent,with a binder of the LUDOX® PX30 type in the form of an aqueous solutionof colloidal silica containing 20 to 40 weight percent of SiO₂. Themould is baked at a temperature of 800-850° C. for 2 hours. Then it isslowly cooled to a temperature of 750° C., held at this temperature for15 minutes, and next poured with an aluminium alloy, which is ahypoeutectic EN AC-AlSi7Mg0.6 silumin at a temperature of 700-720° C.The ceramic mould poured with liquid alloy is after 10 seconds immersedat a speed of 7.5 mm/s in a 20 volume percent aqueous solution of theliquid polymeric quenching agent such as Aqua-Quench 260 at atemperature of 20° C.

The use of an aqueous solution of the liquid polymer makes its reactionwith the ceramic mould and liquid alloy very weak, and the resultingcasting is compact and free from any internal gas-type porosity, whileits outer surface is bright and glossy, with only traces of roughness.

Example 2

A ceramic mould for a tapered casting, as in Example 1, wherein the saidmould is made of an aluminosilicate material in the form of a syntheticfiller under the trade name of Molochite and the LUDOX® PX30 binder, andwherein the said mould is baked at 900° C. for 2 hours, and then cooledto room temperature. After preheating the said mould to a temperature of300° C., it is held at this temperature for 15 minutes and poured withliquid cast MgAl9Zn1 magnesium alloy at a temperature of 690-710° C.Then, after 10 seconds, the mould poured with molten metal is immersedat a constant speed of 7.5 mm/s in a 20 volume percent aqueous solutionof the liquid POLIHARTENOL-E8 polymer at a temperature of 20° C.

After crystallisation and cooling of the alloy in a cooling medium, themould shell does not show any signs of adherence to the outer surface ofthe casting. The casting is compact, free from any external and internalporosity, while its density is close to the theoretical one typical foran alloy of a given chemical composition. When hit with a metal tool,the casting produces a clear metallic sound, typical for gravity diecastings. The outer surface of the sample has a low surface roughnessand is slightly glossy.

Example 3

A ceramic mould of a conical shape based on Molochite as a filler andwith the LUDOX® PX30 binder is baked at a temperature of 950° C. for 3hours and then cooled to a temperature of 700° C., held at thistemperature for 20 minutes and poured at a temperature of 1450° C. withliquid chromium-molybdenum cast iron, containing in weight percent: 3.35C, 0.53 Si, 92 Mn, 9.5 Cr, 0.14 Ni, 1.53 Mo, rest Fe. Then, after 15seconds, the mould with liquid cast iron is immersed at a speed of 5mm/s in an aqueous solution of the liquid THERMISOL QZS 700 polymer at aconcentration of 19.14 volume percent and at a temperature of 50° C.

The directional cooling has resulted in improved compactness: theaverage density of casting ρ=7.51 g/cm³, while the average hardnessvalue is equal to 664 HV. For comparison, the properties of casting madein a sand mould are: ρ=7.45 g/cm³, and the average hardness value equalto 547 HV. The accelerated crystallisation and cooling favour thespheroidisation of primary and eutectic carbides in a ferritic matrix.

Example 4

A ceramic mould made of aluminosilicate filler called Molochite and theLUDOX® PX30 binder based on colloidal silica is baked for 3 hours at900° C. Then the mould is cooled to room temperature and then heated to400° C. The mould is held at this temperature for 30 minutes and pouredwith the liquid AC-AlSi7Mg0.3 alloy at a temperature of 710-740° C.After 15 seconds, the mould poured with the liquid alloy is immersed atan average speed of about 5 mm/s in a tank filled with a polymercoolant, which is a 20 percent aqueous solution of POLIHARTENOL-E8 atroom temperature.

The microstructure of the resulting casting is more refined than themicro-structure of casting solidifying in a self-supported ceramic mouldat a temperature of 400° C. The overall quality of the outer surface ofcasting has been very good in both cases: it has low surface roughness,and is bright and glossy. The casting made by the method according tothe invention is more compact, with no internal defects and, comparedwith the casting made by a traditional technique, depending on wallthickness, has a higher density of from 0.01 to 0.04 g/cm³. Fastercrystallisation has reduced the α_(Al) dendrites of the aluminium solidsolution; particularly visible is the effect of undercooling-relatedmodification of the (α_(Al)+β_(Si)) eutectic. It has also been observedthat in hypoeutectic silumins of the ACAlSi7Mg0.3 type, the DAS or SDASparameters, i.e. the interdendritic spacing of I- and II-order, bettercorrelate with the solidification rate than the grain size. In castingmade from an AlSi7Mg0.3 alloy by the method according to the invention,the average dendrite arm spacing is 40 μm. The increased solidificationrate in casting made according to the invention is reflected in anincrease of the mechanical properties: R_(m)=280 MPa, R_(ρ0.2)=235 MPa,A₅=4.0%, while, for example, in the casting solidifying in a traditionalceramic mould at room temperature, the average dendrite arm spacing is47 μm, and mechanical properties assume the following values: R_(m)=245MPa, R_(ρ0.2)=195 MPa, A₅=2.5%.

Example 5

Three ceramic moulds based on Molochite with the LUDOX® PX30 type binderare baked at a temperature of 900° C. for 2.5 hours and then are cooledto 500° C., held at this temperature for 15 minutes and are next pouredwith liquid copper alloy of the BA1044, B555, or M059 type, according toPN-91/H-87026, overheated by 100-150° C. above the initial meltingpoint. After 30 seconds since the end of the mould cavity filling, themould is immersed at a speed of 8.5 mm/s in a polymer coolant based onPOLIHARTENOL-E8, at a concentration of 15-25volume percent H₂O, at atemperature of 35° C.

The outer surface of copper alloy castings is characterised by lowsurface roughness and is glossy, especially in the case of BA1044aluminium bronze. Rapid directional solidification of copper alloycastings reduces in the microstructure both the grain size and SDAS. Forexample, in B555 alloy, these distances were reduced: from 33 μm—forcastings made by the method according to the invention, to 26 μm forcastings undergoing free cooling in a self-supported molochite mould ata temperature of about 500° C. Cooling conditions had a significanteffect on the hardness of copper alloy castings, and while HV of theBA1044 alloy in a self-supported mould was 232 units, it increased to253 in casting solidifying directionally in the mould placed in a liquidpolymer medium. After rapid directional solidification, a generalincrease was observed in the strength parameters R_(m) and R_(ρ0.2), inductility A₅, and in hardness HV of the examined copper alloys.Additional increase in the mechanical properties is possible afteradjustment of the basic chemical composition, and the application ofmodification-refining and heat treatment.

1. A method for producing investment castings, in ceramic moulds, fromalloys based on Al, Mg, Cu, Zn and Fe, comprising: baking a ceramicmould at a temperature of 800-1000° C. for 2-4 hours; cooling said mouldto a temperature of 20-950° C., and holding said mould at thistemperature for 10-40 minutes; pouring into said mould a liquid alloyoverheated by from 50 to 200° C. above the initial melting point andafter a lapse of 10-100 seconds; and immersing said mould at a fixed orvariable speed in a liquid cooling medium, the said liquid coolingmedium being a 1-99 volume percent aqueous solution of liquid polymer ata temperature of 15-80° C.
 2. A method for producing investment castingsaccording to claim 1, characterised in that the liquid polymer is apolymer of the PAG, or PVP, or ACR, or PEO type.
 3. A method forproducing investment castings according to claim 1, characterised inthat the ceramic moulds made of aluminosilicate or high aluminarefractories, especially based on synthetic sand, for example Molochite,are used.
 4. The method according to claim 1, further comprising:pouring an aluminum alloy into said mould.
 5. The method according toclaim 1, further comprising: pouring a magnesium alloy into said mould.6. The method according to claim 1, further comprising: pouring a copperalloy into said mould.
 7. The method according to claim 1, furthercomprising: pouring an iron alloy into said mould.